In many industrial processes, undesirable excess heat is removed by the use of heat exchangers in which water is used as the heat exchange fluid. Copper and copper-bearing alloys are often used in the fabrication of such heat exchangers, as well as in other parts in contact with the cooling water, such as pump impellers, stators, and valve parts. The cooling fluid is often corrosive towards these metal parts by virtue of containing aggressive ions and by the intentional introduction of oxidizing substances for biological control. The consequences of such corrosion are the loss of metal from the equipment, leading to failure or requiring expensive maintenance, creation of insoluble corrosion product films on the heat exchange surfaces, leading to decreased heat transfer and subsequent loss of productivity, and discharge of copper ions which can then "plate out" on less noble metal surfaces and cause severe galvanic corrosion, a particularly insidious form of corrosion. Also, copper is a toxic substance, and its discharge to the environment is undesirable.
Accordingly, it is common practice to introduce corrosion inhibitors into the cooling water. These materials interact with the metal to directly produce a film which is resistant to corrosion, or to indirectly promote formation of protective films by activating the metal surface so as to form stable oxides or other insoluble salts. However, such protective films are not completely stable, but rather are constantly degrading under the influence of the aggressive conditions in the cooling water. Because of this, a constant supply of corrosion inhibiting substances, sufficient to the purpose, must be maintained in the cooling water. But because many cooling systems are open, a constant depletion of these corrosion inhibiting substances occurs, requiring a continuous addition of fresh corrosion inhibiting substances so as to maintain, within defined limits, a concentration of such corrosion inhibiting substances sufficient to the purpose of maintaining good corrosion inhibition. The need to constantly replace the corrosion inhibiting substances leads to increased costs of operation, and often requires expensive equipment to monitor and regulate the addition of these substances.
Another undesirable feature of the continuous feed requirements of these inhibitors is the continuous discharge of these materials into the environment. Since many of these corrosion inhibiting substances have measurable toxicities for various aquatic species, their continuous discharge presents a chronic hazard to the environment. The benzotriazoles are also somewhat problematic in this regard.
In the treatment of copper-bearing metallurgies an additional complication arises. Unlike the corrosion products of ferrous metals, which quickly form insoluble oxides which will not react further, the corrosion products of copper-bearing metallurgies, namely cupric and cuprous ions, remain soluble and are reactive towards the inhibitors specific for such metals. As a result, the copper-specific inhibitors are further depleted by deactivation. Under certain circumstances, such as acid spills, process leaks, overfeeds of oxidizing biocides, or inadvertent loss of inhibitor feed, the corrosion rate of the copper-bearing metallurgies can increase to such an extent that all the remaining inhibitor is depleted by deactivation. Unless this condition is recognized and specific recovery procedures are instituted, it is clear that no useful effect of additional maintenance dosages of the inhibitor will be obtained since the inhibitor will be deactivated at a rate equal to its addition rate.
Use of substituted benzotriazoles as corrosion inhibitors is a well-known practice. U.S. Pat. No. 4,060,491 relates to the use of 5-alkylbenzotriazoles in lubricants for the reduction of wear of steel surfaces. In U.S. Pat. No. 4,519,928, it is disclosed that N-t-alkylated benzotriazoles are useful for imparting oxidation and corrosion resistance to oleaginous lubricant compositions. British Pat. No. 1,065,995 teaches that 5-alkyl substituted benzotriazoles are effective in reducing corrosion or tarnish of copper items in glycolic solvents or in lubricants, or to resist tarnishing in the presence of atmospheric sulfides. The use of substituted benzotriazoles as metal inactivators in detergent compositions is described in U.S. Pat. No. 2,618,606. Another ferrous metal corrosion inhibitor is claimed in U.S. Pat. No. 3,895,170, in which 1-hydroxy-4(5) substituted benzotriazoles are the objects of the invention.
More directly related to the present invention are the teachings of U.S. Pat. No. 4,406,811, in which benzotriazole or tolyltriazole is combined with other components to form an effective multimetal corrosion inhibitor for aqueous systems.
Japanese Pat. No. 56-142873 relates to a reaction product of alkylbenzotriazoles and phosphonic acids for use in aqueous systems in concentrations of 10-5000 ppm; the object being to improve the dissolution rate of the benzotriazole. Another Japanese patent, No. 57-152476, pertains to the combination of benzotriazoles and cyclic amines for inhibiting metallic corrosion in engine cooling systems, industrial heat exchangers, brake fluids, cutting oils, and glycolic oils.
However, of those disclosures that relate to the inhibition of corrosion of copper-bearing metals in aqueous systems, all require the constant presence of the inhibitor in the aqueous medium. It is clear from the examples provided that the inhibitor must be continuously present in the aqueous phase in order to maintain adequate protection. All of the examples cited fail to address the method of inhibiting corrosion by the formation of a stable and durable inhibiting film which does not require a maintenance level of inhibitor in the aqueous medium.